High frequency amplifier



W. P. MASON HIGH FREQUENCY AMPLIFIER July 24, 1934.

Filed Jan. 26, 1933 rkEQue'A/cr 2st QQEESQ o FREQUENCY INVENTOR W. P. MA SON A TTORNEY Patented July 24, 1934 UNITED STATES HIGH FREQUENCY AMPLIFIER Warren P. Mason, West Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 26 1933, Serial No. 653,620

.5 Claims.

This invention relates to band selective amplifiers for use at high frequencies and more particularly to amplifiers employing vacuum tubes of the screen-grid type having high internal impedance.

The principal object of the invention is to improve the efilciency of high frequency amplifiers ments can not be constructed with characteristic impedances of more than about 40,000 ohms in the transmission range. When higher impedances are attempted the effect of dissipation in the inductance coils greatly reduces the efficiency of transmission in the selected band and also practically destroys the band selectivity. When impedances of the order of 40,000 ohms or less are used, the great disparity of the tube and the circuit impedances results in a large part of the possible amplification being dissipated in the internal resistance of the tube.

In the amplifier circuits of the invention, these losses are avoided by the use of certain types of piezo-electric crystal band-pass filters as the coupling circuits between successive tubes. In

the preferred forms, the impedance branches of the filters comprise piezo-electrio quartz crystals shunted by inductances, the latter being so proportioned and so disposed in the circuit that their dissipation is without effect upon the filter charactenstlcs' I have found that filters of thls type transmission characteristic of the filter as herecan be built to transmit bands of frequencies of widths as great as 10,000 cycles per second at mean frequencies of over 100,000 cycles per second and may have characteristic impedances which closely match the internal impedance of typical screen-grid amplifier tubes. The combination of these filters with screen-grid amplifier tubes, therefore, permits the maximum possible amplification to be obtained while at the same time great sharpness of selectivity is maintained.

The invention will be more fully understood from the following detailed description and from the appended drawing of which;

Fig. 1 shows one form of the invention;

tubes makes it difiicult Figs. 2 to 5, inclusive, are diagrams and curves explanatory of the circuit of Fig. 1; a

Fig. 6 illustrates a modified form of the invention, and

Fig. '7 is a diagram explanatory of Fig. 6.

The amplifier of Fig. 1 is of the push-pull, or balanced, type each of the first two stages comprising a pair of similar screen-grid tubes 10, 10 with their cathodes connected together and to ground and the third stage comprising a pair 5 of three electrode tubes 11, 11' similarly connected. For the sake of simplicity, the circuits for heating the cathodes are omitted and the sources of potential for energizing the grids and the plates are indicatedschematically as batteries. It will be understood, however, that any of the usual arrangements may be used for these purposes.

The successive pairs of tubes are coupled in tandem through similar lattice type filters 14 which will be described in detail later. The grids of the first stage tubes are connected througha transformer 13 to a wave source 12 and the plates of the last stage tubes are connected through a transformer 15 to output terminals 16 and 1'7. 30 Resistances 18 connecting the grids and the cathodes of the second and third stage tubes provide proper resistive termination for the filter for which purpose they should have values equal to the plate circuit resistance of the preceding tube. In most cases, however, they may be omitted without serious eifect upon the amplifier characteristic.

The filters 14 are of the symmetrical lattice type, the line branches comprising piezo-electric quartz crystals X1 and the lattice branches comprising crystals X2 shunted by capacities C2. At each end of the lattice a shunt inductance L and a shunt capacity C are connected which cooperatewith the crystal branches to determine the inafter described. The inductance at the input end of the filter also serves as a plate current supply path for the tubes connected thereto, a tap at the midpoint being provided for connection to the plate current source.

The filter is equivalent in its transmission properties to the symmetrical lattice shown schematically in Fig. 2 in which the line branches comprise the parallel combination of inductance L, capacity C01, and a resonant circuit made up of inductance Lxl and capacity Cxl. The lattice branches comprise a similar combinations of inductance L, capacity C02 and resonant combination 11x2, 0x2. In this equivalent circuit, the crystals X1 have been replaced by the resonant circuit Ln, Ca, and a. shunt capacity which together with the external capacity C makes up the shunt capacity CO1. A similar substitution has also been made for the crystals in the line branches.

The electrical equivalent of the crystal described above has been found to represent the properties of the crystal very accurately over a frequency range up to and well above the first natural resonance of the crystal. For filter purposes the crystals should preferably be of quartz in the form of a rectangular plate cut with its plane perpendicular to the electrical axis and its greater length along the mechanical axis. The electrodes should be applied to faces perpendicular to the electrical axis and the crystal should be mounted so that it is free to vibrate freely by expansion and contraction along the mechanical axis.

For this form of crystal the equivalent electrical coefficients are given in terms of the crystal dimensions by the following formulae:

L, henrics C t farads (1) C, farads I: and f4 and the condition for the existence of a single continuous transmission band is indicated by the coincidence at frequency f: of a resonance of the one impedance with an anti-resonance of the other and a similar coincident at frequency f3. The frequency f: corresponds 'to the resonance frequency of crystal X1 and frequency fa to the resonance of crystal Xi.

The values of the impedance elements of the circuit of Fig. 2 are readily found in terms of the inductance L and the critical frequencies ii to I; by means of the theorem described in an article by R. M. Foster entitled A reactance theorem" Bell System Technical Journal, Vol. III, No. 2, April 1924. For the case illustrated the values are as follows:

The characteristic impedance K of the filter is given by the equation its value being real, or resistive, at frequencies between the band limits an and m4 and imaginary, or reactive, elsewhere. At the band limits the characteristic impedance is infinite and at the near band frequency it has a minimum value Kn given by 1 K 1/ o1 a2( 4'- 1) The variation of the characteristic impedance with frequency is shown in Fig. 4, the dotted portion of the curve representing reactive values and the solid portion resistive values. The horizontal line 21 represents a typical value of the impedances between which the filter should operate, this value preferably being about 20 per cent greater than the minimum value of the characteristic impedance given by Equation 4.

From the foregoing formulae it will be seen that by choosing different values of the critical frequencies f2 and f: a large number of networks may be computed all of which have the same transmission band and the same characteristic impedance at the mean frequency of the band, but have different values of the component impedance elements. The particular values chosen for these frequencies determine to a large extent the attenuation characteristic of the filter at frequencies outside the band and also the phase characteristic within the band. A simple rule for the allocation of the frequencies is to make their values form a geometric series, that is, such that the ratio of each frequency to the next higher frequency is the same in each case. Under this condition a high level of attenuation is maintained at all frequencies outside the band when the shunting inductances are equal. The geometric spacing of the frequencies also corresponds to the condition that the shunt inductances provide the proper amounts of reactance to neutralize the total shunt capacities of the respective branches at the resonance frequencies of the crystals.

In practice, slight deviations from the geometric spacing of the frequencies do not affect the attenuation level outside the band to any great extent and, accordingly. other frequency allocations may be used. When the band width is narrow, of the order of 10 per cent of the mean band frequency, the geometric arrangement will not difier noticeably from a simple uniform spacing of the frequencies at equal intervals. Another rule for the spacing of the frequencies to maintain a high attenuation level and a linear phase characteristic 1,828,454 to H. W. Bode.

The value of the shunt inductances may be determined from the desired characteristic impedance using an alternative form of Equation 4 as follows:

the value of Km being chosen with respect to the terminal impedances between which the filter is to work in the manner already indicated. Having thus determined the value of the shunt inductance the remaining quantities follow from Equation 2.

Since the crystal capacities are very small and is described in U. S. Patent 1 lJu' Lit

described above to pass a band of 10,000 cycles since the .addltionalshunting capacities may be quite small also, it follows from Equation 4 that the mean frequency impedanceof the filter may be made very large. In practice, however, it is necessary to take into account the fact that the inductance coil has some shunt capacity which will appear as part of the effective capacity shunting the crystal thereby reducing the value of. the characteristic impedance. In addition the electrode capacities of the connected vacuum tubes will also be effectively in shunt to the crystal. With suitable coil construction the total external capacity may be kept down to about twenty micromicrofarads which is small enough to permit impedances of several hundred thousand ohms to be obtained.

The magnitudes of the "quantities involved will be illustrated by the following example of a filter designed to couple vacuum tubes having plate resistances of 500,000 ohms and to pass a band of L=0.52 X 10- henries Co1==32.9 10- farads C02=33.35 X 10' farads Cx1=1.468 X 10- farads CX2:1.459X 10- farads Lx1=11.91 henrles Lx2=11.90 henries Choosing athickness of .05 centimeter for each crystal the following values of the crystal dimensions and of the elements of filter as arranged in Fig. 1 are obtained.

For crystal X1 1:0.224 centimeter; 10:0.11 centimeter; t=0.05 centimeter;

for crystal X1.

1:0.223 centimeter; 10:0.11 centimeter; t=0.05 centimeter.

The capacity C in shunt tothe external in ductance L has the value.3 2.7 micro microfarads, and C2 in shunt to crystal X: is 0.45 micro microfaraids. The capacity C will, of course, include the shunt capacity of the inductance and also, the tube capacity, which may amount to a total of about twenty micro microfarads. The use of small variable air condensersin shunt to the coils permits the capacity value to be adjusted to the correct value taking into account these extraneous capacities.

The particular example described above is suitable for use in an intermediate frequency amplifler of a short wave double detection radio receiver. As the frequency is lowered it becomes necessary to usethinner crystals in order that the requisite crystal capacity, determined mainly, by the band width and. the characteristic impedance, can be obtained. It is practicable, however, to use crystals as thin as one-tenth of a millimeter and by increasing the mean frequency characteristic impedance to about 400,000 ohms it becomes possible to constructfilters of the type per second at a mean frequency of about 200,000 cycles per second. Narrower bands can be obtained at still lower frequencies.

The spacing of the resonance and anti-reso-.

' tained as. the result of this is illustrated by curve 22 of Fig. 5. The attenuation peaks of the filter cause the amplification to fall sharply to zero just outside the band limits and the substantial equality of the two branch impedances maintains the attenuation high at all other frequencies outside the band. The two crests of the amplification characteristic occur at the frequencies within the band at which the filter impedance exactly matches the terminal impedances.

An adaptation of the invention to amplifiers of the unbalanced type is shown in Fig. 6 in which a pair of screen grid tubes 10, 10, are coupled by means of an unbalanced high impedance band filter 23. As in Fig. 1. 12 denotes a source of high frequency waves and 13 a transformer coupling the source to the input terminals of the first. stage tube. The output of the second tube is connected to an amplifier detector 24 which may beof conbridged-T type comprising a T-network of equal.

for the purpose of balancing the dissipation in the filter.

The plate current of the first tube is fed through the inductanoes L: and L and to pre vent the plate voltage reaching the electrodes of the second tube blocking condensers 25 and 26 are inserted in the circuit. Additional condensers27 are connected in shunt to the plate current sources to provide low impedance paths for the high frequency currents.

The filter is equivalent in its transmission properties to the symmetrical lattice filter shown schematically in Fig. 7. The line branches of this filter, of which only one is shown, consists of three parallel branches namely, an inductance L corresponding to the series inductance of the T in Fig. 6, a resonant circuit consisting of an inducance /21. and a capacity 20:: corresponding to the resonance of the crystal X1, and a shunt capacity G1 which is equal to the capacity C plus twice the capacityCsi plus twicethe crystal electrode capacity. The line branches will thus be seen to include an impedance equal to half the impedance of crystal X1 together with the other elements mentioned. The lattice branches each include only a simple anti-resonant circuit composed of capacities C in parallel with an inductance of value L+2La and a resistance 21'.

The values of the equivalent lattice elements may be computed in the same manner as in the case of the filter of Fig. 2, the resistance 21' being neglected in this part of the design computation. In general, the filter is subject to the same kind of limitations as to band width and impedance as that of Fig. 1 but since the crystal X1 in the bridged-T arrangement of Fig. 6 has twice the impedance of the crystal equivalent in the lattice it can be constructed with smaller capacities without the minimum thickness limit being exceeded. Consequently, filters of this type can be built for lower frequencies than those of Fig. 1 and filter amplifiers of the type shown in Fig. 6 can be constructed to pass bands of about 10,000 cycles per second at mean frequencies down to about 100,000 cycles per second.

The value of the resistance r in the shunt branch of the filter may be determined as follows: Each of the shunt coils of the equivalent lattice of Fig. 7 will have a certain resistance the effect of which is to diminish the sharpness of selectivity of the filter and to introduce irregularities into its transmission characteristic. If the resistance of inductance L is designated 1'1 and that of coil L2 as r2, the total resistances of the inductive branches of the line and the lattice arms of the filter of Fig. '7 will be re spectively n and T1+2(1'2+1') At the frequencies of the transmission band and over a considerable range on each side thereof these resistances may be replaced by shunt resistances of values respectively, which will be equal when When the resistance r is proportioned in accordance with this formula, the dissipative effects are the same as would be produced by connecting equal high resistances across the input and the output terminals of the filter and removing all dissipation from within the filter. This effect manifestly will appear as a constant loss at all frequencies and will not affect the selectivity.

In Fig. 6 the T-network of inductances L and L2 is made up of three physical inductances. It is, of course, obvious that the shunt inductance Lz may be replaced by mutual inductance between the two series coils in accordance with well known principles.

What is claimed is:

1. In combination, a thermionic amplifier having an anode, a cathode, a control grid, and a screen grid, and a band pass wave filter connected to the output terminals thereof, said wave filter including a. piezo-electric crystal and additional reactances proportioned with respect thereto to provide a single transmission band between preassigned frequencies, and having a characteristic impedance within said band substantially equal to the internal space resistance of said amplifier.

2. An amplifier for signal modulated high frequency waves comprising in combination a screen grid vacuum tube, a band-pass wave filter having its input terminals connected to the anode and cathode respectively of said vacuum tube, and a utilization circuit connected to the output terminals of said filter, said filter including in at least one of its branches a piezo-electric crystal and having an inductance effectively in shunt to said crystal, said crystal and said inductance being proportioned with respect to the other elements of said filter to provide a single transmission band between preassigned frequencies and a characteristic impedance within said band substantially equal to the internal space path resistance of said vacuum tube.

3. An amplifier for signal modulated high frequency currents comprising in combination a pair of thermionic space discharge tubes each having a cathode, an anode, a control grid, and a screen grid, and a band-pass filter having its input terminals connected to the anode and cathode of one of said tubes and its output terminals to the cathode and control electrode of the second of said tubes, said filter comprising a. plurality of impedance branches at least one of which includes a piezo-electric crystal, and an inductance effectively in shunt to said crystal, said crystal and said inductance being proportioned with respect to the other elements of said filter to provide a single pass-band between preassigned frequencies and a characteristic impedance substantially equal to the internal space path resistance of said vacuum tubes.

4. In combination with an amplifying stage comprising a screen grid vacuum tube, a bandpass wave filter having its input terminals connected to the output terminals of said amplifying stage, said filter comprising a plurality of impedance branches each including a piezo-electrio crystal and arranged to form a symmetrical lattice network, an inductance effectively shunting each of said crystals proportioned to cooperate therewith to provide a single transmission band between preassigned frequencies and to make the characteristic impedance of said filter substantially equal to the internal space path impedance of said amplifying stage at the frequencies of the transmission band.

5. A combination in accordance with claim 4 in which the shunting inductances of the filter comprise equal inductances connected respectively between the input and the output terminals of the lattice.

- WARREN P. MASON. 

